Soil Types and Shaking Amplification

One contributor to the site amplification is the velocity at which the rock or soil transmits shear waves (S-waves). Shaking is stronger
where the shear wave velocity is lower. The National Earthquake Hazards Reduction Program (NEHRP) has defined 5 soil types based on
their shear-wave velocity (Vs). We have modified these definitions slightly, based on studies of earthquake damage in the Bay Area.
The modified definitions are as follows: 

          A) Vs > 1500 m/sec (This soil type occurs infrequently in the bay area. We consider it with type B. They are both represented
          by the color blue on the map). Soil type A includes unweathered intrusive igneous rock. Soil types A and B do not contribute
      greatly to shaking amplification. 

          B) 1500 m/sec > Vs > 750 m/sec. Soil type B includes volcanics, most Mesozoic bedrock, and some Franciscan bedrock.
          (Mesozoic rocks are between 245 and 64 million years old. The Franciscan Complex is a Mesozoic unit that is common in the
      Bay Area.) 

          C) 750 m/sec > Vs > 350 m/sec. Soil type C includes some Quaternary (less than 1.8 million years old) sands, sandstones and
          mudstones, some Upper Tertiary (1.8 to 24 million years old) sandstones, mudstones and limestone, some Lower Tertiary (24
      to 64 million years old) mudstones and sandstones, and Franciscan melange and serpentinite. 

         D) 350 m/sec > Vs > 200 m/sec. Soil type D includes some Quaternary muds, sands, gravels, silts and mud. Significant
      amplification of shaking by these soils is generally expected. 

         E) 200 m/sec > Vs. Soil type E includes water-saturated mud and artificial fill. The strongest amplification of shaking due is
         expected for this soil type.